This invention relates generally to drainage systems and, more particularly, to polystyrene beads of novel design for use in preassembled drainage units from which drainage fields such as sewage septic fields may be constricted.
In recent years, preassembled drainage units such as those illustrated in U.S. Pat. No. 5.015,123 have been used for constructing drainage fields for effluent from sewage septic tanks, replacing the traditional plastic pipe and gravel aggregate system. Those preassembled units are generally of two types, one type including a length of perforated distribution polyethylene vent pipe surrounded by a predetermined volume of loose lightweight pieces of plastic aggregate bounded by a perforated sleeve member, the other type being conduitless and merely including the plastic aggregate contained within the sleeve. The manner in which those units are assembled to form drainage fields is described in U.S. Pat. No. 5,015,123 and that description is incorporated herein by reference.
In practice, in the preassembled units, the loose pieces of plastic aggregate are at least 1xe2x80x3 cubes produced from blocks of expanded polystyrene 4xe2x80x2xc3x974xe2x80x2xc3x978xe2x80x2 in size. The blocks are initially cut into 1xe2x80x3 sheets, and the sheets are then dropped onto a hot wire grid and cubed into 1xe2x80x3 cubes. However, the use of cut cubes creates some problems. Cut polystyrene cubes tend to flake and particles of polystyrene fly everywhere. Also, if vehicles drive over the drainage fields, the cut polystyrene cubes tend to collapse and the water filtration slows or stops.
Thus, there is a need for a plastic aggregate which overcomes those problems, and the novel plastic aggregate of this invention satisfies that need.
Accordingly, the primary object of the invention is to provide an expanded plastic bead of novel configuration which is strong and durable, which provides greater void space to promote the flow of waste water through the drainage units, and which provides maximum surface area for bacteria to attach to, the bacteria acting on the sewage effluent, returns it to the soil.
The objectives of the invention are accomplished by providing a configured block-like polystyrene bead made primarily from recycled polystyrene materials of low melt flow to give the bead toughness and durability. The bead is formed by extruding material through an appropriately shaped and sized extrusion die, cutting the material to form beads of a predetermined length, and heating and expanding the beads so that each of its six sides is at least one inch in size. The bead has a longitudinally extending central portion and a plurality of spaced legs, preferably at least three, extending upwardly and three spaced legs extending downwardly from the central portion. A rib projects outwardly from the central portion on each side face of the bead. Channels are formed between adjacent legs and the width of the channels is less than the width of the legs. When contained within a drainage unit, this prevents the legs of one bead from interlocking in the channels of an adjacent bead and maintains maximum void space. Similarly, the ribs on the side faces of the beads maintain spacing between adjacent beads to maximize void space. The large void space allows for maximum flow and storage of waste water. In addition, the novel bead configuration with the legs and channels provides maximum surface area for growth of bacteria which enhances the breakdown of sewage waste.
Other objectives and advantages will become apparent from reading the following description of the invention wherein reference is made to the accompanying drawings.